The invention relates to a film bag having a bag body formed of a synthetic-resin or plastic film. The bag body usually has at least two face panels between which are opposite side gussets. In addition, the film bag can also be provided with a carry handle and a reclosable fastener in the form of a zipper or slide fastener.
In practice, film bags are used for different goods, so that bulk contents are well protected by the film bag body. In particular, film bags can be tightly sealed, resulting not only in improved protection but also a longer storage capability compared with paper packaging. Products such as powdery building materials, cat litter, washing powders or the like can also be effectively protected against moisture, even under nonoptimum storage conditions. Other products, such as foodstuffs and animal feed, stay fresh for a long time due to the airtight packaging.
In practice, film bags are either made directly before filling in a so-called FFS process (form, fill and seal) or as premanufactured bags intended for subsequent filling. The present invention relates in particular to premade film bags that are subsequently, usually after intermediate storage and transportation, filled and tightly sealed.
Different production methods are known for making prefabricated film bags that are subsequently filled and sealed. The whole film bag can be formed from a single film web by folding. Corresponding film bags are disclosed in EP 1 777 167 and EP 0 341 532 (U.S. Pat. No. 5,048,976).
On the other hand, EP 1 541 332 (U.S. Pat. No. 7,775,957) describes a method of making film bags in which a continuous web of bags is formed in such a way that successive film bags have their long sides or longitudinal edges next to one another. Here, the production direction therefore corresponds to a transverse direction of the individual bags. According to EP 1,541,332, a web is initially formed with successive, initially joined bag blanks. A first film web is guided in the production direction. A strip is then introduced with its edges folded over as flaps onto a middle section of the strip. The folded-over edge flaps are usually of equal width so a central gap remains between the two edges after folding.
Pieces of the strip are then placed on the first film web with equal spacing perpendicular to the production direction (that is to say in the bag longitudinal direction) before a second film is introduced in the production direction and set atop the first film web and the folded-over edge flaps. Longitudinal welds are then made perpendicular to the production direction, by means of which, on the one hand, the folded-over flaps are welded to the second film web at their outer edges, and on the other, to the first film web on the opposite side. Finally, individual film bags are separated from the web so formed by cutting in the region of the strips in such a way that two side gussets of successive film bags are formed from one folded strip.
The material used for the bag film enables a comparatively strong and resistive design of the film bag. In practice, use is made, for example, of multilayer laminated bag films that have a sealing layer of polyolefin forming the bag inner face and an outer layer of polyester, in particular polyethylene terephthalate (PET) or biaxially oriented polypropylene (BO-PP), forming the bag outer face.
As well as the advantages of a good protection and a long storage capability, the tight seal due to the film also has disadvantages. In particular, the tight seal prevents equalization of pressure between the interior of the bag and the external environment. Changes in the air pressure can therefore lead to inflation or contraction of the bag. The contents can also build up a superatmospheric or subatmospheric pressure inside the bag during storage. The result can be chemical reactions or even temperature variations, for example, that expand or contract the contents or the air inside the bag.
When a tightly sealed film bag is transported from a packing location at sea level to a region at higher altitude for example, it inflates on account of the lower air pressure. Inflation is also observed at high ambient temperatures when the air in the package expands, the vapor pressure in the package increases and/or the contents outgases. Under certain circumstances, the effects described can lead to damage or at least to an appearance viewed by a user as unfavorable. With an inflated film bag, the risk of it bursting in the event of mechanical loads, for example if it is dropped or thrown, also increases.
Film bags having a valve or that are continuously ventilated in order to prevent inflation are disclosed in the prior art. However, both variants result in a poorer seal of the film bag, and in addition the installation of a valve is laborious.
As well as inflation of the film bag, in practice, contraction occurs when the pressure inside the bag is reduced compared to ambient pressure. As well as the usual air pressure variations, the packing of contents that are initially warm or hot also leads to a frequently undesirable contraction.
When the film bag is provided with a carry handle, the usual materials for the bag body result in the disadvantage that it can tear if tension is suddenly exerted on the carry handle.